In PCM communication systems in which a line is energized with unipolar voltage to transmit messages in a binary code, the distortion coefficient of the line in terms of a ratio of incorrectly and correctly transmitted bits can be determined by generating a succession of binary test words at the transmitting end along with matching reference words at the receiving end and by comparing the incoming test words, bit by bit, with the locally generated reference words. In such a system, the two code-word generators at opposite ends of the line are synchronized with the aid of timing signals extracted from the incoming test words at the receiving end; if a succession of error indications shows that the two generators have fallen out of step, synchronism can be restored by delaying the locally generated reference sequence until the errors disappear.
Recent developments have led to the adoption of bipolar coding designed to minimize the electrical unbalancing of a line, i.e. to suppress the d-c component present in a unipolar signal. Thus, a balanced ternary code known as "AMI" (for "Alternate Mark Inversion") converts the "marks", i.e. the bits of logical value 1, in a binary code word into alternating positive and negative pulses while the intervening "spaces" (i.e. bits of logical value 0) are represented by zero voltage. A more sophisticated code known as "HDB.sub.n " (for "High-Density Bipolar") introduces a so-called violation, i.e. a pulse of the same polarity as an immediately preceding marking pulse, into the bit stream after every n.sup.th space in an unbroken succession of zeros of the original binary message, with relative polarity inversion between consecutive violation pulses; thus, the designation HDB.sub.3 denotes a balanced ternary code in which the line is never de-energized for more than three pulse cycles or bit periods in a row. To preserve the basic balance as well as the distinction between marking and violation pulses, such a code also calls for the retroactive introduction of an additional bit, namely a pulse with the same polarity as the following violation pulse, at the beginning of any four-zero series immediately preceded by a marking pulse whose polarity matches that of the previous violation pulse.
Upon the reconversion of such a ternary code into the corresponding binary pulse sequence at the receiving end, a comparison between the incoming test words and the local reference words no longer insures proper interpretation of an error signal, or of a succession of such signals, as the result of faulty synchronization. Thus, if the errors are due to improper reconversion, the malfunction cannot be remedied by delaying the reference sequence.